1. Field of the Invention
The present invention relates to a servo-control apparatus for motor, servo-controlling a motor by using a DSP as digital control means, and more particularly it relates to a motor servo-control apparatus suitable for controlling a plurality of motors used in an image forming apparatus.
2. Related Background Art
FIGS. 12 and 13 are conventional circuit diagrams for effecting servo-control of a plurality of motors by using a micro computer. Particularly, FIG. 12 is a block diagram showing an entire circuit in which a plurality of motor units 301 having the same construction are connected to a single micro computer 300, and FIG. 13 is a block diagram showing an internal construction of one of the motor units 301.
Now, the conventional servo-control will be described. In FIGS. 12 and 13, there are shown a micro computer 300, motor units 301, a control IC 302, a three-phase motor 303, three hole sensors 304 for detecting a position of a main pole of a rotor, an FG sensor 305 for detecting a pattern magnetized on the rotor and for outputting 36 pulses per one revolution of the motor, an oscillator 306, a current detecting resistor 307, a control portion 308, a driver portion 309, an electric current limiter detecting portion 310, a speed control portion 311, a frequency divider 312, an integration amplifier 313, resistors and capacitors constituting integration filters 314 to 317, a control signal 318 emitted from the micro computer 300 and adapted to drive/stop the motor, and a ready signal 319 which becomes active when the motor reaches a predetermined revolution number.
Next, an operation of the circuit will be described. When a motor driving command is emitted from the micro computer 300 controlling an image forming apparatus through the signal line 318, the control portion 308 detects the position of the main pole of the rotor of the motor 303 by the hole sensors 304 and forms a three-phase exciting pattern so as to rotate the motor in a desired direction and sends an exciting signal to the driver portion 309. On the basis of the exciting signal, the driver portion 309 drives an output transistor (not shown) so that an electric current direction with respect to a coil of the motor is switched to generate desired excitation. On the other hand, when the rotor of the motor 303 is rotated, predetermined pulses are generated by the FG sensor 305 and are sent to the speed control portion 311. In the speed control portion 311, a reference clock formed by the oscillator 306 and the frequency divider 312 is compared with the pulse detected by the FG sensor 305, and the difference therebetween is outputted.
Incidentally, the reference clock is set to obtain a target revolution number (number of revolutions) of the motor. Namely, when the FG sensor outputs 30 pulses per one revolution of the motor, in order to rotate the motor at 600 rpm, the reference clock of 300 Hz (=(600/60)xc3x9730) may be given.
The difference with respect to the target speed obtained by the speed control portion 311 is integrated by the integration amplifier 313, and a result is sent to the driver portion 309. In this case, gain and a phase compensation value are determined by the resistors and capacitors 314 to 317. These constants are referred to as servo constants.
Further, in the driver portion 309 for the motor of the conventional image forming apparatus, a transistor of bipolar type is used. Thus, since heat loss of the driver portion is great, a radiator plate is provided. Further, in order to reduce heat generation due to such heat loss as much as possible, the efficiency of the motor must be increased so that the desired power can be obtained with the least electric power. To this end, a brushless motor of the outer rotor type having good efficiency is used.
As mentioned above, in the conventional circuit arrangement, the motor is controlled by sending only stop/start signals to the motor units 301 from the micro computer 300, and a servo-control loop is formed in each motor unit 301. The reason for this is that, since the processing ability of the conventional micro computer is limited, servo-control must be effected in each motor unit 301. As the processing ability of the micro computer or a DSP (digital signal processor) has been improved, servo-control for the motors has been able to be effected by the micro computer or the DSP itself. Further, due to an increase in processing ability of the DSP, a plurality of motors have been able to be servo-controlled independently.
As a result, in place of the above-mentioned conventional circuit arrangement, it has been considered to provide a circuit having motors servo-controlled by the DSP. Such a circuit will be explained herein below. FIGS. 14 and 15 are views showing such a circuit. Particularly, FIG. 14 is a block diagram showing an entire circuit in which a plurality of motor units are connected to a single DSP, and FIG. 15 is a block diagram showing the internal construction of one of the motor units.
In FIGS. 14 and 15, there are shown a DSP 501 serving to control six motors 505, motor units 502 each including a drive circuit, a driver 504, a three-phase DC brushless motor 505, a charge pump circuit 401 for generating gate voltage for N-chMOS of the driver 504, pre-driver circuits 402 to 407, exciting switching signals 408 to 413, a current sense signal 414, hole sensor signals 415 to 417, an MR sensor signal 418, hole sensor amplifiers 419 to 421, an MR sensor amplifier 422, N-chMOS transistors (driver portions) 515 to 520, a current detecting resistor 521, U-phase output 522 connected to a U-phase coil of the motor, V-phase output 523 connected to a V-shape coil, W-phase output 524 connected to a W-phase coil, hole sensors 525 to 527, an MR sensor 528, and a serial communication bus 532 for effecting communication with a control CPU (not shown) of the image forming apparatus.
Next, an operation of this servo-control circuit will be described. First of all, when a motor drive command is transmitted from the CPU through the serial communication line 532, the DSP 501 ascertains the position of the rotor detected by the hole sensors 525 to 527 on the basis of the hole sensor signals 415 to 417 and determines the switching timing so as to obtain the desired rotation and effects control on the basis of the switching signals 408 to 413 to give a desired rotational direction and a desired electric current to the motor.
Namely, the N-chMOS transistors 515 to 520 are switched to give the desired rotational direction, and the N-chMOS transistors 515, 517, 519 are PWM-switched to cause the desired electric current to flow into the coil of the motor. In this case, the gate voltages of the N-chMOS transistors 515, 517, 519 are increased to Vcc+10V by the charge pump circuit 401.
For example, when the DSP 501 ascertains the rotor position of the motor on the basis of the hole sensor signals 415 to 417 amplified by the hole sensor amplifiers 419 to 421 and the hole sensors 525 to 527 and switches the direction of the electric current from the U-phase 522 to the W-phase 523 to obtain the desired rotational direction, the pre-drivers 402 to 407 turn ON the N-chMOS transistors 515, 518 and turn OFF the transistors 516, 517, 519, 520. As a result, an electric current path extends from Vcc to the current detecting resistor 521 through the transistor 515, U-phase output 522, V-phase output 523 and transistor 518, thereby generating a magnetic force in the desired coil. In this case, the PWM signal given by the DSP 501 is composed or combined with the switching signal 408, so that the N-chMOS transistor 515 is PWM-controlled by the pre-driver 402.
Accordingly, ON-duty electric current defined by the PWM signal flows from the U-phase to the V-phase. In this way, the motor is subjected to exciting switching control for switching the electric current to U, V, W-phase to rotate the rotor in the desired rotational direction, thereby generating torque by relative electromagnetic action between the main pole magnet (not shown) and the coil.
When the motor is subjected to the exciting switching control in this way to rotate the rotor, a pre-set MR sensor magnetizing pattern is detected by the MR sensor 528, thereby outputting 360 pulses per one revolution. Namely, a signal having frequency corresponding to the revolution number of the motor is obtained, and this signal is inputted to the DSP 501 as the MR sensor signal 418 through the amplifier 422.
The DSP 501 measures a pulse interval of the MR sensor signal 418 and seeks the speed (rad/s) of the motor and compares the motor speed with a target control speed and performs PI filter (not shown) and gain added calculation (not shown) to derive PWM pulse width and combines the pulse width with the switching signals 408, 410, 412 to control the current to be supplied to the motor coil, thereby controlling the motor to rotate at the target speed.
In this way, the DSP 501 effects the switching of the output stage N-chMOS transistor by generating the PWM signal and combining it with the switching signals, thereby performing the servo-control to rotate the motor at the desired number of revolutions. On the other hand, the position of the main pole is detected by the hole sensors 525 to 527, and the switching control is performed on the basis of the hole sensor signals 415 to 417 to rotate the rotor in the desired rotational direction. Further, the current flowing through the motor is detected by the electric current detecting resistor 521, and, there is provided protecting means for limiting the electric current if the current greater than a predetermined value flows.
In the conventional motor servo-control apparatus explained in connection with FIGS. 12 and 13, the micro computer effects drive/stop control of the drive motors, and each drive motor has serve-control IC and the serve-control is effected in each motor unit. Namely, the feedback loop is closed in the motor unit. Further, stability of the servo-control of each motor is determined by constants, i.e., serve constants of the resistors and capacitors connected to the integration amplifier of the circuit. Namely, these servo constants were required to be set so that the motors be rotated most stably and accurately under every conditions in consideration of load inertia and load torque.
As a result, when the above-mentioned conventional motor servo-control apparatuses are used as various drive means of an image forming apparatus of electrophotographic type having a cartridge integrally including toner and a photosensitive drum, if the load inertia and the torque of the drive motor for driving the photosensitive drum is greatly changed in dependence upon a difference in toner capacity, a difference in the kind of toner, or a difference in the cartridge used, there arose a problem that stable servo-control could not achieved under all conditions.
Further, in a color image forming apparatus, in order to enhance a glossy property of the apparatus, there is provided a glossy print mode in which a recording paper is conveyed at a speed slower than a normal recording paper conveying speed so that a time period during which the recording paper is passed through a fixing device is increased to improve toner fusion. Thus, the drive motors must be controlled with plural speeds, and, if the speed control range is wide, stable servo-control cannot be obtained by only one servo constant system.
As one method for solving this problem, there has been proposed a technique in which a plurality of integration amplifiers are provided and the amplifiers are switched in accordance with conditions. However, this technique has a disadvantage that the cost is increased considerably.
Further, in general, speed detecting means for the servo motor is provided on the motor itself, for the purpose of improvement in rotational accuracy and stability of the rotor of the motor.
In an image forming apparatus using such a motor, for example, if fluctuation in rotation due to fluctuation in load is caused on a shaft of a photosensitive drum, when such fluctuation in rotation can be corrected by the servo-control of the motor, the fluctuation in rotation of the drum shaft can be reduced, thereby obtaining good image quality. However, since the conventional motor is a DC bruchless motor of outer rotor type having a main pole magnet of the rotating rotor, inertia of the rotor is great. Accordingly, the fluctuation in rotation generated on the drum shaft is hard to be transmitted to a drive shaft of the drum drive motor. As a result, even if the servo-control of the drum drive motor is performed accurately, unevenness in rotation of the drum cannot be improved, with the result that deterioration of image quality cannot be reduced.
Further, there is an image forming apparatus using a stepping motor in place of the DC brushless motor. However, the stepping motor has low efficiency in comparison with the DC brushless motor. Thus, particularly in the color image forming apparatus having a plurality of motors, if all of the motors are stepping motors, the load on the power supply of the apparatus becomes great, thereby increasing the total cost of the apparatus considerably. Further, since the stepping motor generates great vibration during step driving, when the plurality of stepping motors are used, the noise generated by the apparatus becomes great.
As an apparatus normally considered to eliminate the above-mentioned disadvantages of the conventional motor servo-control device shown in FIGS. 12 and 13, there is the motor servo-control apparatus explained in connection with FIGS. 14 and 15. In this servo-control apparatus, since the DSP performs all of the phase switching control, speed control, and electric current limiting control, if the number of the motor units connected to the DSP is increased, adequate processing cannot be achieved. Further, since a great number of signal lines are required between the DSP and the motor units (for example, eleven signal lines for each motor unit), the number of input/output pins is increased, with the result that the control ability of the interfaces may deteriorate. Further, in the electric current limitation, when the electric current detection voltage is sent from the motor unit to the DSP, if a distance between the motor unit and the DSP is long, noise will be generated.
When digital servo-control of the plural motors is effected by the DSP, if control timings for the motors overlap for the number of motors to be controlled, since the servo-control period of the motor does not become constant, unevenness in rotation of the motor will occur.
Further, the servo constant of the motor to be servo-controlled is determined by the torque constant, the inertia, and the coil resistance of the motor to be connected.
In such a construction, particularly when motors are purchased from different companies, the servo constant must be set so that stable servo-control can be achieved under all conditions of torque constants, inertia, and coil resistances of the motors to be used.
For example, the inertia of a motor of the outer-rotor type differs greatly from the inertia of a motor of the inner-rotor type. In such motors having different inertia, the setting of a proper servo constant is limited.
Namely, if the servo constant is set to match the motor of the outer rotor type to enhance the servo stability of the motor of the outer-rotor type, when the motor of the inner-rotor type is used, servo stability of such a motor will be worsened.
Thus, in the past, when the servo constant was selected, it was difficult to enhance the stability of servo-control of all of the motors to be used.
The present invention aims to eliminate the above-mentioned conventional drawbacks and an object of the present invention is to provide a motor servo-control apparatus which has a construction suitable for effecting servo-control by using a DSP as digital control means.
Another object of the present invention is to provide a motor servo-control apparatus in which a plurality of motors used in an image forming apparatus are controlled collectively by using a DSP as digital control means.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.